Attention deficit hyperactivity disorder (ADHD) is a serious, common, and chronic behavioral syndrome characterized by impaired attention, impulsivity, and excessive motor activity. It is a common reason for service referrals and is a risk factor for multiple poor outcomes. The proposed study would examine attentional and related inhibitory processes in the primarily hyperactive/impulsive (ADD+H) and primarily inattentive (ADD-H) subtypes at two developmental periods. A secondary goal of the proposed study, intended to increase the breadth of research, is to explore genetic contributions involved in the etiology of the two ADHD subtypes. Children will be recruited from local schools and pediatric clinics. Young adults will be recruited from the Disabilities Resource Center located on the Michigan State University Campus and through newspaper ads/fliers distributed in local communities. ADHD and comorbid diagnoses will be based on parent/ teacher and self report (questionnaires and a structured interview) for children and adults, respectively, according to DSM-IV criteria. Participants will complete a computer-generated attentional orienting task to measure the automatic and effortful allocation of visual attention and related inhibitory processes. Check cell samples for genetic analysis will be collected. The proposed cognitive neuroscience and genetic approach to understanding ADHD has the potential to better identify the primary cognitive dysfunction in ADHD as well as inform an etiologic model of these dysfunctions, and thus improve nosology, leading to the eventual development of better remedial programs.